In 3GPP LTE (3rd Generation Partnership Project Long Term Evolution), as one of the next-generation cellular systems, investigations into SON (Self Organizing Network) are going on from the perspective of reducing the operation expenditure (OPEX).
SON has such functions as
self-configuration;
self-optimization; and
self-healing.
Techniques for achieving the different objectives are being considered (Non-Patent Documents 1 and 2).
In Non-Patent Document 2, as one of SON techniques there is listed radio parameter optimization with a view to accomplish respective objectives, such as maximizing the throughput, minimizing the delay or maximizing the coverage.
Examples of radio parameters include the transmission power of a radio base station (sometimes abbreviated to a ‘base station’), an antenna tilt angle of a base station, neighbor cell information and handover parameters. One or more of these radio parameters are changed as necessary to accomplish the above objectives.
In the following, attention is focused on the neighbor cell information, out of the above radio parameters, which is related to the present invention.
In LTE, there is stipulated an ANR (Automatic Neighbor Relation) function as one of methods for optimizing the neighbor cell list (Non-Patent Document 1). This ANR function renders it possible to automatically add a cell (not a target for a measurement report) that is not recognized by a base station of a serving cell to a neighbor cell list. The following describes the ANR function that generates a neighbor cell list of the same frequency in the LTE system, as an example, with reference to FIG. 18.
In FIG. 18, it is assumed that a terminal (UE: user equipment) is in a cell and communicates with a base station 1 (eNB1), and that a cell2 is adjacent to the cell1.
It is assumed that the presence of an eNB2 (cell2) is not recognized by the eNB1.
The UE detects that a received power of a down-link reference signal (pilot signal) received from the cell2 is greater than or equal to a given threshold value ((Measurement (Phy CID=5) <step 0>. The UE notifies the eNB1 of the physical cell ID of the cell2 (Physical Cell ID: Phy CID=5) and the received power value with a measurement report (Measurement report) <step 1>.
Since the eNB1 is informed by this report of the presence of Phy CID (=5) the eNB1 does not recognize, the eNB1 requests a global cell ID (Global CID) from the UE as the further information regarding the relevant cell (Report Global CID request) <step 2>.
The UE reads the broadcast information of the cell2 broadcasted over BCH (Broadcast CHannel), detects the Global CID of the cell2 (=19) <step 3>, and makes a report to the eNB1 (Report Global CID=19) <step 4>.
The eNB1 then establishes connection with the eNB2 (X2 setup) and exchanges with the eNB2 information such as served cell lists (Served cell lists) mutually. X2 is a logical connection (interface) between eNB's.
The information necessary for optimizing the neighbor cell list may thus be obtained. The base station determines whether or not to add the new cell to the neighbor cell list, depending on whether or not the value of the received power of the downlink signal of another cell reported by a terminal is greater than or equal to a given threshold value and whether or not the number of times of the report exceeds a given number (Patent Document 1).
FIG. 19 is a diagram illustrating the ANR function of the eNB and the NRT (see Non-Patent Document 1). Specifically, FIG. 19 shows an example of a neighbor cell list retained by an eNB. The neighbor cell list includes an index, identifier of a target cell (Target Cell ID: TCI), associated with the index, and attributes for each of the target cells (“No Remove”, “No HO” and “No X2”). It is noted that HO means Hand Over and X2 means an interface between base stations in LTE. Such neighbor cell list is termed a neighbor relation table (NRT). The NRT, a neighbor cell list including an index, an identifier of a target cell associated with the index (TCIs) and attributes of each target cell, as one entry, is used as part of the ANR (Automatic Neighbor Relation) function. In case a check (check mark) is inserted in “No Remove”, a radio base station never deletes the cell from the neighbor cell list (delete inhibited). In case a check is inserted in “No HO”, the radio base station does not use the cell in question as an object in the hand-over (use inhibited). In case a check is inserted in “No X2”, X2 is not used (is not to be used) to perform specific procedures for the base station that manages the cell.
Not only the large improvement of peak throughput compared with WCDMA (Wideband Code Division Multiple Access) but also a method of improving cell edge throughput are investigated for the LTE. One of the investigations is ICIC (Inter Cell Interference Coordination).
The ICIC is a technique of exchanging information on the neighbor cells, for example, resource usage ratio and traffic load status, between the base stations, to control the radio resources, so as to reduce a neighbor cell interference. The ICIC may be said to be SON in the broad sense of the term (Non-Patent Document 1).
The investigation of LTE Advanced, in which the LTE is further advanced, is also commenced, and investigation of CoMP transmission/reception (Coordinated MultiPoint transmission and reception) is being conducted as a means for further improving the cell edge throughput.
The CoMP is a technique in which a plurality of base stations simultaneously sends down-link data to a specified terminal or simultaneously receives uplink data from a specified terminal (Non-Patent Document 3).
As regards the CoMP, a serving cell (Serving Cell), a cooperating set (CoMP cooperating set) and so forth are defined.
The serving cell is a cell that transmits control information to a terminal over a down-link control channel (PDCCH).
The cooperating set (CoMP cooperating set) is a set of cells that directly or indirectly take part in transmitting data to a terminal over the down-link control channel (Physical Down Link Shared CHannel or PDSCH).